BURNUP CREDIT ANALYSIS SEQUENCE

ISG8 highlights the need for applicants employing burnup credit in criticality safety assessments to account for the axial and horizontal variation of the burnup within a spent fuel assembly. In practice, the axial burnup variation (e.g., the axial burnup profile) is commonly modeled in a criticality calculation using a finite number of axial segments or zones (10 to 20 is typical) to represent the burnup profile, each zone having a uniform average burnup for that segment. Consequently, implementation of burnup credit using this approach requires separate fuel depletion calculations for each axial zone, and the subsequent application of these spent fuel compositions in the criticality safety analysis. Implementation of this approach requires that numerous spent fuel depletion calculations must be performed, and potentially large amounts of data must be managed, converted, and transferred between the depletion and criticality codes.

0

Axial depth of CR insertion (distance from top of active fuel, cm)

∆k[k(CRs) - k(no_CRs)] actinide only

FIG. 5. Impact of CR insertion during irradiation on SNF in the GBC-32 cask.

To simplify this analysis process and assist the NRC staff in their review of criticality safety assessments of transport and storage casks that apply burnup credit, a new SCALE control sequence, STARBUCS (Standardized Analysis of Reactivity for Burnup Credit using SCALE) has been created [15]. STARBUCS automates the generation of axially varying isotopic compositions in a spent fuel assembly, and applies the assembly compositions in a three-dimensional (3-D) Monte Carlo analysis of the assembly in a cask environment. The STARBUCS control sequence uses the new ORIGEN-ARP methodology [16] of SCALE to perform automated and rapid depletion calculations to generate spent fuel isotopic inventories in each axially-varying burnup zone of a fuel assembly. The analyst need only specify the average assembly irradiation history, the axially varying burnup profile, the actinides and, optionally, the fission products that are to be credited in the criticality analysis. An arbitrary number of axial zones may be employed, or the user may select from several pre-defined profiles. This series of calculations is used to generate a comprehensive set of spent fuel nuclide compositions for each axial zone of the assembly. The STARBUCS sequence uses the SNF inventories provided for each zone to automatically prepare cross sections for the criticality analysis. A 3-D KENO V.a criticality calculation is performed using cask geometry specifications provided by the user. Isotopic correction factors (ICFs) may also be applied to correct the criticality calculation for known bias and/or uncertainty in the prediction of the isotopic concentrations.

This new STARBUCS sequence has been used at ORNL to support the study of the impact of various assumptions that might be applied in the development of a loading curve. Figure 6 illustrates three loading curves highlighted against the 1998 inventory of U.S. discharged fuel.

The loading curves show how the assumptions relative to selected nuclides and associated ICFs can lead to significant increases in the spent fuel inventory that can be loaded in a burnup credit cask. The curves indicate that, as discharge burnups and initial enrichments increase, efforts to incorporate fission products and/or reduce the ICFs will be needed to assure a burnup credit cask can carry a significant portion of the fuel anticipated for future discharge.

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FIG. 6. Illustrative loading curves for GBC-32 cask shown with PWR SNF discharge data through 1998 (numbers in legend indicate number of assemblies). Dashed lines represent current burnup and enrichment limits of ISG8. ICF refers to the Isotopic Correction Factors.

7. SUMMARY

The technical bases needed to help improve and expand the U.S. regulatory guidance for burnup credit in transportation casks have been developed at ORNL under the direction of the U.S. NRC research staff. The goal has been to develop criteria and/or recommendations that are technically credible, practical, and cost effective while maintaining needed safety margins. The technical work performed at ORNL is now undergoing final review by NRC staff and it is anticipated that changes to the recommendations of ISG8 will be forthcoming.

ACKNOWLEDGEMENTS

The authors want to recognize the numerous personnel from NRC, ORNL, and industry who have posed questions, generated suggestions, and provided technical information that have aided in the progress of this research. Particular thanks to M. D. DeHart of ORNL and F. Eltawila, R. Y. Lee, D. D. Ebert, D. E. Carlson, and C. J. Withee of the NRC.

REFERENCES

[1] “Spent Fuel Project Office Interim Staff Guidance - 8, Rev. 1 - Limited Burnup Credit,”

U.S. Nuclear Regulatory Commission, July 30, 1999.

[2] Standard Review Plan for Transportation Packages for Spent Nuclear Fuel – Final Report, NUREG-1617, U.S. Nuclear Regulatory Commission, March 2000.

[3] PARKS, C.V., DeHART, M.D., WAGNER, J.C., Review and Prioritization of Technical Issues Related to Burnup Credit for LWR Fuel, NUREG/CR-6665 (ORNL/TM-1999/303), U.S. Nuclear Regulatory Commission, Oak Ridge National Laboratory, February 2000.

[4] WAGNER, J.C., “Addressing the Axial Burnup Distribution in PWR Burnup Credit Criticality Safety,” 35218.pdf in Proc. of 2001 ANS Embedded Topical Meeting on Practical Implementation of Nuclear Criticality Safety, November 11–15, 2001, Reno, NV (2001). [ANS Order No.: 700284; ISBN: 0-89448-659-4].

[5] CACCIAPOUTI, R.J., Van VOLKINBURG, S., Axial Burnup Profile Database for Pressurized Water Reactors, YAEC-1937, Yankee Atomic Electric Company, May 1997.

[6] PARISH, T.A., CHEN, C.H., “Bounding Axial Profile Analysis for the Topical Report Database,” Nuclear Engineering Dept., Texas A&M University, March 1997.

[7] WAGNER, J. C., Computational Benchmark for Estimation of Reactivity Margin from Fission Products and Minor Actinides in PWR Burnup Credit, NUREG/CR-6747 (ORNL/TM-2000/306), U.S. Nuclear Regulatory Commission, Oak Ridge National Laboratory, October 2001.

[8] WAGNER, J. C., PARKS, C.V., “Impact of Burnable Poison Rods on PWR Burnup Credit Criticality Safety Analyses,” Trans. Am. Nucl. Soc. 83, 130–134, 2000. Also see, WAGNER, J.C., PARKS, C.V., Parametric Study of the Effect of Burnable Poison Rods for PWR Burnup Credit, NUREG/CR-6761 (ORNL/TM-2000/373), U.S. Nuclear Regulatory Commission, Oak Ridge National Laboratory, March 2002.

[9] SANDERS, C.E., WAGNER, J.C., “Impact of Integral Burnable Absorbers on PWR Burnup Credit Criticality Safety Analyses,” 35235.pdf in Proc. of 2001 ANS Embedded Topical Meeting on Practical Implementation of Nuclear Criticality Safety, November 11–15, 2001, Reno, NV (2001). [ANS Order No.: 700284; ISBN: 0-89448-659-4]. Also see, SANDERS, C.E., WAGNER, J.C., Study of the Effect of Integral Burnable Absorbers for PWR Burnup Credit, NUREG/CR-6760 (ORNL/TM-2000/321), U.S. Nuclear Regulatory Commission, Oak Ridge National Laboratory, March 2002.

[10] SANDERS, C.E., WAGNER, J.C., “Parametric Study of Control Rod Exposure for PWR Burnup Credit Criticality Safety Analyses,” 35281.pdf in Proc. of 2001 ANS Embedded Topical Meeting on Practical Implementation of Nuclear Criticality Safety, November 11–15, 2001, Reno, NV (2001). [ANS Order No.: 700284; ISBN: 0-89448-659-4]. Also see, SANDERS, C.E., WAGNER, J.C., Parametric Study of the Effect of Control Rods for PWR Burnup Credit, NUREG/CR-6759 (ORNL/TM-2001/69), U.S. Nuclear Regulatory Commission, Oak Ridge National Laboratory, February 2002.

[11] THIOLLAY, N., CHAUVIN, J.P., ROQUE, B., SANTAMARINA, A., PAVAGEAU, J., HUDELOT, J. P., TOUBON, H., “Burnup Credit for Fission Product Nuclides in PWR (UO2) Spent Fuels,” presented at the Sixth International Conference on Nuclear Criticality Safety, ICNC 99, September 20–24, 1999, Versailles, France.

[12] GAULD, I.C., PARKS, C.V., Review of Technical Issues Related to Predicting Isotopic Compositions and Source Terms for High-Burnup LWR Fuel, NUREG/CR-6701 (ORNL/TM-2000/277), U.S. Nuclear Regulatory Commission, Oak Ridge National Laboratory, January 2001.

[13] NAKAHARA, Y., SUYAMA, K., SUZAKI, T., Technical Development on Burnup Credit for Spent LWR Fuels, Japan Atomic Energy Research Institute, Tokai Research Institute, JAERI-Tech 2000-071, September 2000 (in Japanese).

[14] GAULD, I.C., PARKS, C.V., “Strategies for Applying Isotopic Uncertainties in Burnup Credit,” presented at The IAEA Technical Committee Meeting on Requirements, Practices, and Developments in Burnup Credit Applications, April 22–26, 2002, Madrid, Spain.

[15] GAULD, I.C., SANDERS, C.E., “Development and Applications of a Prototypic SCALE Control Module for Automated Burnup Credit Analysis,” 35238.pdf in Proc. of 2001 ANS Embedded Topical Meeting on Practical Implementation of Nuclear Criticality Safety, November 11–15, 2001, Reno, NV. [ANS Order No.: 700284;

ISBN: 0-89448-659-4]. Also see, GAULD, I.C., BOWMAN, S.M., STARBUCS: A Prototypic SCALE Control Module for Automated Criticality Safety Analyses Using Burnup Credit, NUREG/CR-6748 (ORNL/TM-2001/33), U.S. Nuclear Regulatory Commission, Oak Ridge National Laboratory, October 2001.

[16] LEAL, L.C., HERMANN, O.W., BOWMAN, S.M., PARKS, C.V., “Automatic Rapid Process for the Generation of Problem-Dependent SAS2H/ORIGEN-S Cross-Section Libraries,” Nucl. Technol. 127, 1–23, July 1999.